The bifunctional modification reagent N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP) has been used to link two proteins together through a disulfide bond. The reagent is reacted with the first protein to introduce an active disulfide-containing group in the modification step. A second protein, which contains a free thiol group, is then added to form a disulfide bond between the two proteins in the conjugation step. Many derivatives of SPDP and imide versions of SPDP have been described (U.S. Pat. No. 4,563,304; J. Carlsson et al. 173 Biochem. J. 723-737 (1978); Goff D. A., Carroll, S. F. 1 BioConjugate Chem. 381-386 (1990); L. Delprino et al. 82 J. Pharm. Sci. 506-512 (1993); S. Arpicco et al., 8 BioConjugate Chem 327-337 (1997)).
Conjugates of cell-binding agents with highly cytotoxic drugs have been described (U.S. Pat. Nos. 5,208,020, 5,416,064; 5,475,092, 5,585,499, 6,436,931, 6,372,738 and 6,340,701; R. V. J. Chari et al., 52 Cancer Res. 127-131 (1992)). In these conjugates, the cell-binding agents are first modified with a bifunctional agent such as SPDP, SPP or SMCC to introduce an active disulfide or a maleimido moiety. Reaction with a thiol-containing cytotoxic drug provides a conjugate in which the cell-binding agent, such as a monoclonal antibody, and drug are linked via disulfide bonds or thioether bonds.
Heterobifunctional cross-linkers comprising a nitropyridyldithio, dinitropyridyldithio, N.N-dialkylcarboxamidopyridyldithio or di-(N.N-dialkylcarboxamido)pyridyldithio group and a reactive carboxylic ester group such as a N-succinimidyl ester group or a N-sulfosuccinimidyl ester group have been described (U.S. Pat. No. 6,913,748). The presence of a N-sulfosuccinimidyl group was claimed to provide higher aqueous solubility to these cross-linkers. However, once the cell-binding agent has been reacted with these cross-linkers, the N-sulfosuccinimidyl group is displaced and the solubility advantage is lost, both for the modified cell-binding agent and its drug conjugate. Since cytotoxic drugs used in cell-binding agent-drug conjugates are often only sparingly soluble in aqueous solutions, it is often difficult to link a sufficient number of drug molecules to the cell-binding agent and still maintain aqueous solubility. In addition, reactions have to be conducted in dilute solutions, which are cumbersome to scale up because of the need to use large volumes of solution.